Difficulty in preservation of sunscreens is a known fact. Different UV-filters have an induced effect on bacteria growth and can even exacerbate contamination of the sunscreen formulation.
Each molecule has a polarity value that can be scaled in many ways. One of the most popular polarity scale is the LogP (Partition coefficient).
Preservatives also have a LogP value that can anticipate their distribution in a cosmetic emulsion.
Non-polar preservatives (LogP >1) will have a tendency to migrate into the organic phase, therefore, deserting the water phase – i.e. the scene of the antimicrobial activity.
Sharon Laboratories are presenting SharoSENSE™ Plus: a preservative solution platform with an ideal polarity index to feet high SPF formulations, providing broad spectrum protection at a low level of use. This platform is free-from parabens, formaldehyde donors and Phenoxyethanol.